It is known in the art to employ a nuclear reactor fuel assembly wherein at least some of the fuel elements are provided with spacer members, each formed as a wire arranged in a helical line on the lateral surface of each of said fuel elements and adjoining, in the contact planes, the fuel elements in immediate proximity to those fuel elements which carry said spacer members. This practice is used in most operating fast reactors with a sodium coolant.
The spacer member used herein is a structural element designed for space fixation of fuel elements.
In the fuel assembly of this type of reactor, the spacer member is formed as a single wire fixed on the ends of each fuel element, and forcibly wound in a helical line about the lateral surface thereof. Each fuel element of the fuel assembly is fixed at a required number of points, vertically and perimetrically, through contact with the spacer members disposed on the adjacent fuel elements.
It is obvious from the above that with such a design of the fuel assembly, a modification of the principle is possible whereby only some, but not all, of the fuel elements are provided with spacer members.
The spacer member formed as a single wire helically arranged on the lateral surface of the fuel element is of fairly simple design, but one which nevertheless provides for the required tightness for the packing of the fuel elements in the fuel assembly and for their secure fixation, as well as for satisfactory thermal and hydraulic characteristics of the fuel assembly as a whole. These advantages account for the wide popularity of the foregoing fuel assembly design in the world reactor-manufacturing industry.
As far as high-power fast reactors are concerned, however, the core elements are exposed to entirely different conditions and altogether new operating factors are involved. Thus, exposure to high integrated neutron fluxes (on the order of 10.sup.23 1/sq.cm. and higher), typical of such reactors, causes the jackets of the fuel elements to swell to a considerable degree, with their outer diameter increasing by as much as several percent (.DELTA. d/d.sub.o). In the above-described fuel assembly, the resultant lateral deformation of the fuel elements will cause an inadmissible degree of deformation of the assembly housing and induce additional contact stresses in the jackets of the fuel elements contacting one another by way of the highly rigid spacer members, with the result that the possible service life of the fuel elements will be severely shortened, the fuel cycle costs raised and the general reactor economics adversely affected.